The key point of this question is to identify the phrase that describes a sudden increase. The correct answer is B. upsurge, which means a sudden increase in something. Option A. dressed in is a phrase used to describe wearing clothes and is unrelated to increases. Option C. flood into means to enter a place in large numbers suddenly, but it does not specifically describe an increase in quantity or intensity. Option D. sweep across refers to something becoming very popular quickly, not a sudden increase in quantity or intensity.